1.Field of the Invention
The present invention relates to non-coherent, frequency agile, radar receivers and, more particularly, to frequency control mechanisms for such receivers.
2. Description of the Prior Art
Non-coherent frequency agile pulse radars are well known to those skilled in the art. In the radar art, it is well understood that targets disposed at certain aspect angles with respect to transmitted pulses of selected frequencies will produce no receivable echo at the radar antenna, thereby permitting the target to go undetected. To overcome this problem, frequency agile pulse radars have been developed which transmit pulses of variable radio frequencies, so that the radar set will not operate continuously at a selected frequency where a target disposed at a particular aspect angle would provide no echo to the receiver. The rate of variation of the radio frequency is referred to as the agility modulation rate and, typically, varies in a sinusoidal fashion.
Operating limitations of the radar receiver components require that the amplification and further processing of the received echo signals be done at intermediate frequencies rather than the radio frequencies of signals provided by the transmitter and received as echos. The conversion of the received signals from a radio frequency to an intermediate frequency is accomplished by mixing the received signals with the output of a local oscillator having a radio frequency which differs from the transmitted radio frequency by the amount of the intermediate frequency. In non-coherent radars, the transmitter signal and the local oscillator signal are generated from separate, independent frequency sources in contrast with coherent radars in which the transmitter signal and the local oscillator signal are generated from the same frequency source.
Since the radio frequency of the transmitted signals is controllably varied, the radio frequency of the local oscillator must also be frequency variable in the same manner as the frequency of the transmitted signals to maintain a constant intermediate frequency. Moreover, since the intermediate frequency must be within the bandwidth of a filter whose bandwidth is matched to the transmitter pulse width to achieve an optimum signal-to-noise ratio for the receiver, the radio frequency of the local oscillator must track the radio frequency of the transmitter output signal within predetermined limits.
The prior art mechanisms for controlling the local oscillator frequency of frequency agile pulse radars have included an electromechanical frequency transducer cooperating with a sample and hold circuit to provide a modulating signal which controls the radio frequency of the local oscillator output signal in response to the radio frequency of the next pulse to be provided by the transmitter. A manual adjustment is provided to control the gain of the modulating signal which is applied to the local oscillator for manually tuning the automatic frequency control mechanism. A fast automatic frequency control loop corrects for errors in the manually tuned modulating signal.
The basic problems with the prior art mechanism were that the manual gain adjustment for the modulating signal was accurate only for the mechanical and electrical transfer functions of the electromechanical frequency transducer and the sample and hold circuit which existed at the time at which the manual adjustment was made. However, these transfer functions were subject to variations due to mechanical wear, aging and ambient temperature variations. The fast automatic frequency control loop corrected the radio frequency of the local oscillator on a pulse-by-pulse basis. However, the accuracy of the correction depended on the various automatic frequency control loop parameters, and the error correction of the fast automatic control loop was found to be relatively sensitive to variations in loop gain parameters. Moreover, variation in the fast automatic control loop parameters would leave a residual correction error of a magnitude which depended on the magnitude of the modulating signal errors and on the automatic frequency control loop parameter variation. The residual error enhanced by the modulating signal errors and the sensitivity of the fast automatic frequency control loop to variations in gain parameters, necessitated frequent manual adjustment of the automatic frequency control and resulted in reliability performance and maintenance requirements which were unacceptable for some applications.